Improve type-checking in OutcomeException

[nicoddemus]

Fix #5578

[asottile]

[asottile]

hmmmm though test_module_level_skip_error (the failing test) seems to maybe already provide this functionality -- just it's not applied to all places (?)

[nicoddemus]

hmmmm though test_module_level_skip_error (the failing test) seems to maybe already provide this functionality -- just it's not applied to all places (?)

Yeah changed the test slightly:

import pytest
pytest.skip(reason='some reason')

Will show the original message.

Any use of pytest.skip, pytest.xfail, etc, will now show the new message about expecting a string and suggesting a mark.

[codecov]

Codecov Report

Merging #5580 into master will increase coverage by 1.17%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #5580      +/-   ##
==========================================
+ Coverage   94.73%   95.91%   +1.17%     
==========================================
  Files         117      117              
  Lines       25713    25721       +8     
  Branches     2493     2493              
==========================================
+ Hits        24359    24670     +311     
+ Misses       1036      744     -292     
+ Partials      318      307      -11

Impacted Files	Coverage Δ
testing/test_skipping.py	97.98% <ø> (ø)	⬆️
src/_pytest/outcomes.py	98.46% <100%> (+3.07%)	⬆️
testing/test_runner.py	96.12% <87.5%> (-0.16%)	⬇️
src/_pytest/pytester.py	85.03% <0%> (-4.89%)	⬇️
testing/test_assertrewrite.py	83.26% <0%> (-1.42%)	⬇️
testing/test_pytester.py	89.27% <0%> (+0.57%)	⬆️
src/_pytest/capture.py	94.73% <0%> (+1.31%)	⬆️
testing/test_terminal.py	99.84% <0%> (+1.4%)	⬆️
src/_pytest/terminal.py	92.6% <0%> (+1.84%)	⬆️
src/_pytest/doctest.py	96.62% <0%> (+2.24%)	⬆️
... and 3 more

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[asottile]